Not every manual needs all of these, and most manuals use only two or
three at most. The present manual, for example, has two indices: a
concept index and an @-command index (that is actually the function
index but is called a command index in the chapter heading).

You are not required to use the predefined indices strictly for their
canonical purposes. For example, suppose you wish to index some C
preprocessor macros. You could put them in the function index along
with actual functions, just by writing @findex commands for
them; then, when you print the “Function Index” as an unnumbered
chapter, you could give it the title ‘Function and Macro Index’ and
all will be consistent for the reader.

On the other hand, it is best not to stray too far from the meaning of
the predefined indices. Otherwise, in the event that your text is
combined with other text from other manuals, the index entries will
not match up. Instead, define your own new index (see New Indices).

We recommend having a single index in the final document whenever
possible, however many source indices you use, since then readers have
only one place to look. Two or more source indices can be combined
into one output index using the @synindex or
@syncodeindex commands (see Combining Indices).